Natural rubber (NR) is obtained from the white latex (latex milk) of particular plants encountered in the tropical climate zone. This latex milk is coagulated to obtain commercially available natural rubber, which finds various uses in large amounts in the rubber-processing industry. For example, in 2005, 8 703 000 t of natural rubber were produced and 8 777 000 t of natural rubber were consumed, of which approximately 70% of the consumption is in the automotive tire sector [source: Römpp, Chemielexikon online, Version 3.1].
The physical and chemical properties of the different natural rubber types are determined by their particular regions of origin and processing or production methods.
An essential criterion in this regard is the production process of the natural rubber latex. This allows the natural rubber types to be divided into two classes, a first class, 1, and a second class, 2.
Class 1:
The rubber types of Class 1 are produced by the conventional process in which the coagulum slabs obtained from latex are processed further directly without comminution. The natural rubber of Class 1 is therefore not subjected to any mechanical stress in the production process and is usually dried at temperatures less than 70° C. As a result, it retains a relatively high molecular weight, which has clear advantages in the dynamic performance of the vulcanizates comprising this natural rubber. A significant disadvantage is, however, the commercial packaging form as “sheets”, which make processibility significantly more difficult. Class 1 includes “Ribbed Smoked Sheets” (RSS), “Air Dried Sheets” (ADS), “UnSmoked Sheets” (USS) and “Pale Crepe”.
Class 2:
The natural rubber of Class 2 is produced by the “crumb process”, wherein the rubber is crushed to crumbs by means of rollers with addition of castor oil, or by mechanical comminution in granulators. The drying, which follows various washing processes, takes place at temperatures up to 130° C. The mechanical process gives rise to lower molecular weights, which have an adverse effect on the dynamic behavior of the corresponding vulcanizates. However, natural rubber of Class 2 is packaged in processing-friendly bales, which ensure significantly improved processibility. Class 2 includes “Standard That Rubber” (STR), “Standard Malaysian Rubber” (SMR) and “Standard Indonesian Rubber” (SIR).
Frequent attempts have been made to influence the properties of the natural rubber according to the criteria desired in each case. As representative examples of further publications on this subject, the following should be mentioned here:
(D1) EP 0 950 485 A1; (D2) DE 199 13 047 B4; and, (D3) EP 1 120 429 A1 (U.S. Pat. No. 6,410,606).
EP 0 950 485 A1 discloses a process for producing natural rubber, and natural rubber thus obtained. Here, before or after a one-stage or multistage drying treatment of the natural rubber, a viscosity stabilizer is added. The usual problems with the drying conditions described in EP 0 950 485 A1 should be alleviated in order to obtain a natural rubber which has a high molecular weight and a low gel content.
The drying treatment results in an amount of gel in the natural rubber of 10% or less and a molecular weight retention of 85% or more.
DE 199 13 047 B4 describes a process for masticating natural rubber in a ramless internal mixer. The purpose of masticating natural rubber is to perform a cleavage of the long hydrocarbon chains in the rubber in order to lower the hardness of the rubber or to make it more easily deformable.
U.S. Pat. No. 6,410,606 discloses a process for producing natural rubber for pressure-sensitive adhesives. To this end, the molecular weight of a non-masticated natural rubber is reduced by using a “peptizer”, while the non-masticated natural rubber is dissolved in an organic solvent.